Nitric acid esters of cellulose and method of preparation



ited States atent 2,776,965 Patented Jan. 8, 1957 NITRIC ACID ESTERS OF CELLULOSE AND METHOD OF PREPARATION No Drawing. Application March 28, 1956, Serial No. 574,388

17 Claims. (Cl. 260-220) This invention relates to new nitric acid esters of cellulose and their preparation; more particularly this invention relates to new nitric acid esters of cellulose free of unstable sulfur compounds and to a method for preparing these esters employing nitrating mixtures containing without causing considerable hydrolysis of the nitrate groups; hence, even the stabilized nitrocellulose of industry still contains traces of unstable sulfur compounds. The stabilization treatment aims to remove suflicient unstable sulfate esters so that the final product will meet at least minimum heat stability requirements as measured by the 134.5 C. heat test.

"Moreover, nitrating mixtures containing nitric acidsulfuric acid-water penetrate into the cellulose relatively slowly. Therefore, it is necessary to prepare the cellulose for nitration with extreme care. Cotton linters must be thoroughly fiulfed up and must be substantially free of all closely packed aggregates of fibers such as balls, pills and the like. Wood pulp must be shredded completely to thoroughly open up the fiber aggregates.

Unless this is very carefully and thoroughly done the nitration will be nonuniform and the final product will not dissolve completely in solvents to form smooth, clear solutions which are necessary to commercial utilization.

Hence it is apparent that the present commercial process for preparing nitrocellulose requires very carefulcontrol and treatment of the cellulose supply to insure uniform products and requires a tedious and lengthy stabilization treatment to produce a stable product.

Much effort has been devoted to develop a practical and economical process for nitrating cellulose which would overcome the recognized disadvantages of the present commercial process involving mixed nitric-sulfuric nitrating mixtures. Heretofore, all such proposed processes have been neither practical nor have they produced acceptable products of commercial utility due to nonuniform nitration resulting in products having poor solubility characteristics.

Now in accordance with this invention novel nitric acid esters of cellulose are produced by reacting cellulose with a nitrating mixture containing essentially nitric acid, magnesium nitrate and water, the ratio of magnesium nitrateto water being at least about 12:1 and notmore, than about 2.211 by 'weightand said nitric acid being present in an amount sufiicient to yield a nitric acid ester of cellulose having a nitrogen content of from about 11% to about 13.5 by weight.

iii

More particularly, novel nitric acid esters of cellulose are produced by reacting cellulose with a nitrating mixture containing essentially between about 45% and about 94% nitric acid, between about 3.3% and-about 34% magnesium nitrate, and between about 2.7% and about water by weight, the ratio of magnesium nitrate to Water being at least about 12:1 and not more than about 2.221. 7

A more specific nitrating mixture suitable for preparing most types of nitrocellulose contains essentially between about 45% and about 80% nitric acid, between about 10% and about 34% magnesium nitrate, and between about 7% and about 21% water by weight, the ratio of magnesium nitrate to water being at least about 1.2:1 and not more than about 2.2:1. Since the nitrating mixtures of this invention contain no sulfuric acid, there are no unstable sulfate esters to contend with, and hence the resulting nitric acid esters do not require a long and tedious stabilization treatment following nitration to produce stable products. The nitric acid esters formed, after removal of the spent nitrating mixture, are stable for about minutes in the standard 134.5 C. heat test Without the necessity of any subsequent stabilization treatment.

Within the operative range set forth above there is a preferred range particularly well adapted for commercial practice wherein the ratio of magnesium nitrate to water is at least about 13:1 and not more than about 2:1, and wherein the nitrating mixture is free of undissolved magnesium nitrate.

A particular embodiment of this invention involves immersing cellulose in a bath of nitrating mixture containing essentially between about and about 80% nitric acid, between about 10% and about 34% magnesium nitrate and between about 7% and about 21% water by weight, the ratio of magnesium nitrate to water being at least about 1.3:1 and not more than about 2:1, said nitrating mixture being free of undissolved magnesium nitrate, the ratio of said nitrating mixture to cellulose being at least about 20:1 by weight, nitrating the cellulose at a temperature between about 15 C.'and

about C. for at. least about 8 minutes and until' nitration is substantially complete, and recovering uniformly substituted nitric acid esters of cellulose substantially completely soluble in acetone to form smooth, clear solutions. I

In a preferred embodiment of the invention, after nitration the spent nitrating mixture is removed from the resulting nitric acid esters, and stable nitric acid esters of cellulose are recovered directly, without subsequent stabilization treatment. Following nitration and removal of spent nitrating mixture, the nitric acid esters may be subjected to subsequent conventional and well-known treat ment for viscosity control, bleaching, dehydration, and the like as desired. I I t The process in accordance with this invention is: particularly well adapted for continuous operation because the nitrating mixture penetrates even dense aggregates of, fibers such as sheeted linters or sheeted wood pulp rapidly and uniformly, and hence nitration is rapid and uniform. This is in marked contrast to the present commercial method of making nitrocellulose employing mixed nitric-sulfuric acids, which has never been success- "fully adapted to continuous operation, clue principally to the necessity for having the cellulose thoroughly fiuffed or completely shredded in order to obtain uniform nitration. a

The new products of this invention are nitric acid esters of cellulose containing between about 11% and about 13.5% nitrogen in the substituent nitrate groups. They are free of unstable sulfur compounds, stable for about 15 minutes in the 134.5 C. heat test, contain detectable amounts of magnesium nitrate less than about 70 parts per million by weight, based on the weight of Mes. 5901 .9 912: 16??? b we ht- Ihs nitt had a stability of about 15 minutes in the 134.5 test and formed a clear, smooth solution in acetone.

the product. ljreferred products are uniformly substitiited," 'e ingf substantially 'comple'tely'solubl'e in acetone 5 to form smooth, clear'solutions.

""fflie following examples setforth some specific embodimerits of thisinyenti'onfbut these are not t ebl 0on A series of nitrocelluloses was prepared following substantially th'e ieeearfre set forth in Example 1. Table l lists pertinent'data for these examples including comd as ii an t nve'ntion. P IJ fh m atm u es, mtra ca 1ms..a ltem- 1 m peratures, type cellulose employed, ratioof n trating. 1111xu t c l ose p t 1. p n acet n solub hty. a A mtrating mixture was prepared WhlCh contained stabil ty n the 134,5,"v (3. heat test.

Table 1 Nitrating Composition, Percent by' Nitra- Ratio .1 a; 1;; Wt. a tion Nitra- Nitretlng Stability 1 Example Temtion Type Cellulose Mixture; Percent Acetone Solu- 134.5 C. MW pera- Time, Cellulose Nitrogen 'bility (Solution Heat test Nitric Magnesium Water N10; ture,0. Minutes (By, 1 Appearance) .(Minutes) Acid Nitrate weight) 50. 00 23.30 70 50 45 Flufied Bulk Linters 54 18 50.00 27.30 10 70 40 11 Shredded'wdodpulm. 54 12+ 50.00 31.72 1828 50 do 54. 12+ 60.00 24.40 15 70 50 10 do 5 15 54. 00 20.00 17 00 50 45 Flufied Bulk Linters. 54' 14 50. 00 32. 70 17 50 25 Shreddedwood pulp 54 12+ 57. 30 19. 27 13 41 50 54 15 69. 73 18.12 12 13 50 54 12+ 58.91 27.45 13 63 50 54. 12+ 69. 74 20. 00 10. 24 50 54 12+ 75. 20 15.80 0; 00 50 55 12+ The designation 12+ means'that the sample was still stable at the end of 12 minutes and the test was stopped at this point.

62.56% nitric acid, 22.52% magnesium nitrate, 14.91% EXAMPLE. 14v water, and 0.01% N203 by Weight. The N203 was only an incidental ingredient in the'nitrating mixture, being present in'the concentrated nitric acid employed. It is Welhrecognized in the art that concentrated nitric acid often contains'small. percentages of N203. The mixture was brought to 503C; and '1 part by weight of shredded wood pulp' driedrl hour at 100 C. was immersed or 40 Q. as mastere it; Table 2following.

dippedinto 54' parts byweightof. the nitrating mixture w 7 n H 7 with agitation to form a slurry. Theslurry; of cellulose TemperatuIepLNitratmn, C. PereentNitrogenin was nitrated ;at abput '50.? 3C; for 10 minutes while agitheNitrocellulose tating, after'which the slurry of nitrocellulose in the spent nitrating mixture was run into a centrifuge. As nauch as possible of the spent nitrating mixture was separated :7

from the fibrous nitrocellulose by centrifugal force, and 11490 the nitrocellulese was then drowned in a large excess water about oq parts water to 1 part nitrocellulose. After draining off the drowning bath, the nitrocellulose was washedwith fresh water untilffreeof acid, The watejpwet nitrgeellulose was then dried 'ina. stream of. t' b tt nd't e tsl lliil ifid. trocellulose was foundto be stablefor algout 15 minutes in the standard134.'5'Cheatjtest. 'lt dissolv ed substantially com- 55 pletely'jn aceton jjto fqrrrr a smooth, clear solution, thus dmoristra'tingihat it w as uniform ly substituted. The nitrogen content determined by the standard nitrometer Int-amass'asam leanea for 1 h011rat 105 C, was

xswus;

Table 3- found to be 12.08%, by weight,

50 Temperature of Nitrat1on, 0. Percent Nitrogen in EXAMPLE 2 the Nitrocellulose A nitrating mixture was prepared. which contained 50 i 6 5.26%"-'ni tr ic acid,"20.'35% magnesiur'nfnitrate, 14.34% 00""" "I: 12:07 wateryand0.05%"N2Q3. This mixture was broughtto g3" j f v 50" and 1"part 'offair-dryfsheeted wood pulp about 5 i 0.05 vinchthick cut into 3-inch squares was'immers'ed or dipped 1'nto66 j parts 'by w eight of the jnitr'a'tinig mixture \i'rithfagitationf The 'cellulosewas 'nitrated at about .50" Cr'for a-p'eriod of 20 minutes"withflagitation,jaftei 'fwhich EXAMPLem the'nitrating'"mixture was drained otf;' an d the sheets of.

nitrocellulose were drowned in a large excess of water, about 60 parts water to 1 m nitrocellulose. After draining ofi' the drowning bath, the nitrocellulose was washed with fresh water until free 505cm andwas dried ina stre'arr of air at about 60 C, The nitrogen content following.

procedur'e'in" Example}, e ployinga ni tratingtmixture r Ztediin Table; 4

A series of nitrocelluloses was prepared follgwing the Table 4 Percent Nitrogen in the Nitrocellulose Time of Nitration in'Miuutes EXAMPLE 17 EXAMPLES 18-23 A series of nitrocelluloses was prepared following substantially the procedure set forth in Example 1. Table 5 lists pertinent data for these examples including composition of the nitrating mixtures, nitration times and temperatures, type of cellulose employed, ratio of nitrating mixture to cellulose, andpercent nitrogen in the nitrocellulose product. p

nitration at equilibrium being governed primarily by the composition of the nitrating mixture. It has been further established that at any given nitric acid content within the limits set forth hereinabove the nitrogen content of the resulting nitrocellulose increases with decreasing water content, in general a decrease of 0.1% in the water content of the nitrating mixture causing a corresponding increase of about 0.05% nitrogen in the nitrocellulose, and vice versa. Obviously, of course, for each change in water content there will necessarily be a corresponding inverse change in the magnesium nitrate content of the nitrating mixture. Similarly, it has been found that at any given water content within the limits set forth hereinabove the nitrogen content of the resulting nitrocellulose decreases With increasing nitric acid content, in general an increase of 1% in the nitric acid content of the nitrating mixture causing a corresponding decrease of about 0.1% nitrogen in the nitrocellulose, and vice versa. Here, likewise, for each change in nitric acid content there will necessarily be a corresponding inverse change in the magnesium. nitrate content of the nitrating mixture.

Hence, it is obvious that there are numerous nitrating mixtures within the operating limits set forth for preparing any patricular nitrocellulose having a preselected nitrogen content. The particular nitrating mixture to be employed will therefore be largely a matter of choice governed primarily by economic considerations.

Preferred nitrating mixtures in accordance with this invention. are free of undissolved magnesium nitrate at the nitrating temperature selected, and preferably such solutions should not be supersaturated with respect to magnesium nitrate. Additionally, in preferred nitrating Table 5 Nitrating Composition, Percent by Wt. Ratio Ni- Ratio Mag- Nitration N itration tratiug Percent nesiuui Example Tempera- Time Type Cellulose Mixture: Nitrogen Nitrate to Nitric Magnesium Water N 0 ture, 0. Minutes Cellulose Water (By Acid Nitrate (By weight) weight) 84. 80 9.13 6.00 0.07 50 10 Shredded wood pulp 32 13.38 1. 52 89. 33 5. 78 4. 75 0. 14 50 10 d0 32 13. 36 1. 22 84. S0 9. 13 6. 00 0. 07 50 10 d0 50 13. 57 1. 52 79. 76 11. 84 .8. 37 0. 03 4O Sheeted 1 Wood pulp tablets 10 12. 59 l. 41 93, 62 3. 63 2. 65 U. 10 45 -d0 1 6 12.76 1. 37 90. 47 5. 56 '3. 92 0. 05 30 lo 1 32 13. 46 1. 42

1 The sheeted wood pulp tablets were approximately "fio inch long x z inch wide x 0.05 inch thick.

The nitrating mixtures in accordance with this inven- 5O mixtures the ratio of magnesium nitrate to water is at tion contain three essential ingredients, namely, nitric acid, magnesium nitrate and water, the ratio of magnesium nitrate to water being at least about 1.2: 1 and not more than about 22:1, and the nitric acid being present in an amount suificient to yield a nitric acid ester of cellulose having a nitrogen content of from about 11% to about 13.5 by weight. It will be understood, of course, that the sum of the three essential components will constitute substantially 100% of the nitrating mixture, any N203 being only an incidental ingredient in the nitrating mixture, since it is well recognized that concentrated nitric acid often contains small percentages of N203, usually on the order of 0.1% or less. Although for most purposes the nitrating mixtures of this invention will usually contain between about 45% and about 94% nitric acid, between about 3.3% and about 34% magnesium nitrate, and between about'2.7% and about 21% water by weight, it is to be understood that the invention is not to be construed as limited in this respect, since the important feature is the ratio of magnesium nitrate to water as set forth hereinabovc, nitric acid being present in an amount suflicient to yield a nitric acid ester of cellulose having a nitrogen content between about 11% and about 13.5% by weight.

It has been found that nitration in accordance with this invention is an equilibrium reaction, the extent of p least about 1.3 :1 and not more than about 2:1 by weight.

A convenient method for preparing the nitrating mixtures of this invention is as follows: magnesium carbonate is slurried with water to form a thick paste, nitric acid of 98%+ strength is then added slowly until the solution is just acid to litmus paper. The solution is then boiled to remove water until the desired magnesium nitrate concentation is obtained. The magnesium nitrate solution is then weighed, cooled to about C. and the requisite quantity of strongnitric acid, 98%+ strength of known composition, is weighed in, additional Water, if necessary, is added, and the mixture is stirred until homogeneous. Magnesium oxide or magnesium hydroxide can be substituted for the magnesium carbonate.

Another convenient method consists in melting commercial crystals of magnesium nitrate hexahydrate, Mg(NO3)2.6H20, and boiling until the desired concentration of magnesium nitrate is obtained. The solution is then weighed, cooled to about 80 C. and the requisite quantity of strong nitric acid, 98%+ strength of known composition, and additional water, if necessary, are added and the mixture stirred until homogeneous.

A convenient method of analysis for checking the composition of nitrating mixtures consists in determining total acidity by titration with sodium hydroxide, determining nitrogen trioxide by titration with potassium permangacomputedby difference. Total acidity can'beconsistently checked within 0.06% and magnesium nitrate within 0.10%by this method.

Any'of the usual commercial forms of cellulose can beemployed in this invention including cotton, cotton linters, wood pulp, regenerated cellulose, and the like.

This can'be'in any form including fibers, staple fibers, filaments, small shreds, chips, shredded linters, shredded wod pulp, granules, sheets, finely ground, fluffed bulk fibers, and the like. It isflpresen tly preferred'to employ shredde dor fiutfed wood pulp or shredded or flu'ffed cotton linters or-mixtures of these since these forms of chemical cellulose are particularly well adapted to present commercial nitrocellulose facilities.

It is emphasized-that cellulose can react with any quantity ofthe nitrating mixtures of this invention to form nitric acid esters of cellulose. However, in reacting cellulose with the nitrating mixtures of this invention nitric acid is consumed and water is formed. From a practical consideration it is desirable to employ an excess of nitrating mixture over the theoretical stoichiometric requirements. With thecustomary nitrating equipment in commonuse in the industry and employing the customary forms of chemical cellulose, it is presently preferred to employ at leastabout parts of nitrating mixture to 1 part of cellulose by weight so that the water formed in the reaction will not unduly-dilute the nitrating mixture, but it is to be understood that theinvention is not limited in this respect. With other for-ms of cellulose and/or other conditions of nitration less than 20 parts nitrating mixture to 1 part cellulose are quite feasible as illustrated by the examples. It will be apparent that larger quantities can be employed, such as 30, 40, 50, 60, 70, or even more parts per part of cellulose, as desired. However, the economics ofthe process will usually govern the ratio of nitrating mixture to cellulose, it being most economical to nitrate'with thelowest ratio that will give a high quality product.

'A Wide range of temperatures can be employed in practicing this invention. For practical reasons, however, it is not desirable to employ temperatures below about 15 C. or higher than about 70 C. Below about 15 C. the reaction becomes too slow to be economically attractive, and above about 70 C. the nitrocellulose tends to crumble so that mechanical losses in the. process become excessive. A preferred range of temperatures lies between about 40 C. and about 60 C., with temperatures of about 50 C. being particularly suitable.

The nitration reaction in accordance with this invention is-very rapid, a major portion of the substituent nitrate groups often being introduced within a matter of two minutes or less. However, a uniformly substituted nitrocellulose in equilibrium with the nitrating mixture usually requires nitration for a longer period of time depending primarily upon the temperature of nitration and the formof cellulose employed. In any case nitration will ordinarily be continued until nitration is substantially complete and the resulting nitrocellulose is in equilibrium with the nitrating mixture. With the customary shredded wood pulp or shredded linters at temperatures above about 30 C. it has been found that at least about 3 minutes is required to obtain a uniformly substituted nitrocellulose. Under similar conditions the commercial process employing mixed nitric-sulfuric acids requires at-least about 18 minutes to obtain uniform nitration. Denser forms of cellulose such as sheets, granules and the like require longer nitration times to achieve a uniformly substituted product, in some instances as much as minutes or more, under similar temperature conditions' to-achieve uniform substitution. Since the rate of nitration .is slower atlower temperatures it follows that longer nitration times are required at lowertemp'eratures.

Agitation during the reactionhastens rapid and uniform wetting of 'all parts of the cellulose with the nitrating mixture, and hence is preferred whenever possible. For eeonomicreasons nitration time will ordinarily be held to a minimum consistent with obtaining a uniformly substituted product, and for any particular case this can readily be established by the simple expedient of determining nitrogen content of the nitrocellulose and testing solubility of the product in acetone.

Following nitration it is necessary to remove the spent nitrating mixture from the resulting nitrocellulose substantially completely in order to recover a usable product. This can .be accomplished by draining off as much as possible of the spent nitrating mixture by gravity drainage, by suction, by centrifugation or similar-means. This leaves a cake or mat of nitrocellulose still wet with adhering spent nitrating mixture which must be removed. The wet cake .or-mat of nitrocellulose is then broken up and drowned in a large excess of water with agitation. The objective of-this drowning operation is to dilute very rapidly and substantially the :adhering spent nitrating mixture remaining on'the nitrocellulose and thusprevent hydrolytic damage to the nitrocellulose. Ordinarily or more parts of water to 1 part nitrocellulose is employed for drowning the nitrocellulose. The drowning bath is then drawn off by any suitable means and the nitrocellulose is then-washed with fresh-water until tests for acidity show that substantially all nitrating mixture has been removed. Washing can be-accomplished by any of the well-known methods for-washing a solid with a liquid, including 'displacement or spray washingwith gravity, suction, -or centrifugation, 'decantation and the like. Washing-is continued until substantially all of the adhering nitrating mixture has been washed out as determined by test on the wash liquor.

' liquid such as ethyl alcohol, for example, before use.

In accordance with this invention novel nitric acid esters are prepared which contain between about 11% and about 13.5% nitrogen in the substituent nitrate groups. These nitrocelluloses are free of unstable sulfur compounds since there-are no sulfur-containing compounds employed in the nitrating mixtures, and they are stable for about 15 minutes. in the standard and wellknown 134.5" C.-heat test. Thesenitrocellulosesusually containdetectable-amounts of magnesium nitrate less than about 70 parts per million by weight based on the weight of :the nitrocellulose. Preferred .nitrocelluloses in :accordance with this invention are uniformly substituted as'evid'encedby -the fact that they-dissolve substantially completelyin acetone -to 'form'v clear, smooth solutions.

This :invention'has several notable advantages over the-well-known commercial process forg-producing nitrocellulosewhich employs mixed nitric-sulfuric'acids. To begin with the nitrating mixtures'of .thisrinvention penetrate and wet even relatively dense aggregates of .cellulose :particles rapidly and uniformly in contrast to mixed nitric-sulfuric acidwhich requires that the cellulose'be very -.carefully-shredded, fluifed and opened up and made free :of. closely packed aggregates of fibers such as pills, balls and the like. The nitration':-reaction of this -"inventionis -rnu'ch more rapidthan the welllcnowmcommercia'l'process. --'Fhe process -o this invencoatings, moistureproof coatings, coated fabrics, ad-

hesives, explosives, and many others.

The 134.5 C. heat test referred to herein in the specification and claims is described in A. S. T. M. D 301-33 Part II of the Book of 1946 on page 1086 with revisions on page 1712.

. This is a continuation-in-part of, our copending application, Serial No. 264,854, filed January 3, 1952, now abandonded.

What we claim and desire to protect by Letters Patent 1s:

1. A process for preparing nitric acid esters of cellulose which comprises reacting cellulose with an excess over theoretical stoichiornetric requirements of nitrating mixture containing essentially nitric acid, magnesium nitrate and water, the ratio of magnesium nitrate to Water being at least about 1.2:1 and not more than about 2.2:1 by weight, and said nitric acid being present in an amount suificient to yield a nitric acid ester of cellulose having a nitrogen content of from about 11% to about 13.5% by weight.

2. A process for preparing nitric acid esters of cellulose which comprises reacting cellulose with an excess over theoretical stoichiometric requirements of nitrating mixture containing essentially between about 45% and about 94% nitric acid, between about 3.3% and about 34% magnesium nitrate, and between about 2.7% and about 21% water by weight, the ratio of magnesium nitrate to water being at least about 12:1 and not more than about 22:1.

3. A process in accordance with claim 2 wherein the ratio of nitrating mixture to cellulose is at least about 20:1 by weight.

4. A process in accordance with claim 2 wherein the ratio of magnesium nitrate to water is at least about 1.3:1 and not more than about 2.021, the nitrating mixture being substantially free of undissolved magnesium nitrate.

5. A process in accordance with claim 2 wherein the cellulose is nitrated at a temperature between about 15 -C. and about 70 C. until nitration is substantially complete and the resulting nitrocellulose is in equilibrium with the spent nitrating mixture.

6. A process for preparing nitric acid esters of cellulose which comprises reacting cellulose with an excess over theoretical stoichiometric requirements of nitrating mixture containing essentially between about 45% and about 80% nitric acid, between about 10% and about 34% magnesium nitrate and between about 7% and about 21% water by weight, the ratio of magnesium nitrate to water being at least about 1.221 and not more than about 2.221.

7. A process for preparing nitric acid esters of cellulose which comprises reacting cellulose with a nitrating mixture containing essentially between about 45% and about 80% nitric acid, between about 10% and about 34% magnesium nitrate, and between about 7% and about 21% water by weight, theratio of magnesium nitrate to water being at least about 1.2:1 and not more than about 2.2:1, the ratio of said nitrating mixture to cellulose being at least about :1 by weight.

8. A process for preparing nitric acid esters of cellulose which comprises reacting cellulose with an excess over theoretical stoichiometric requirements of nitrating mixture containing essentially between about 45% and about 80% nitric acid, between about 10% and about 34% magnesium nitrate, and between about 7% and about 21% water, the ratio of magnesium nitrate to water being at least about 13:1 and not more than about 2.0:1, said nitrating mixture being substantially free of undissolved magnesium nitrate.

9. A process for preparing nitric acid esters of cellulose which comprises reacting cellulose with an excess over theoretical stoicbiometric requirements of nitrating mixture containing essentially between about 45 and about 80% nitric acid, between about 10% and about 34% magnesium nitrate, and between about 7% and about 21% Water by weight, the ratio of magnesium nitrate to water being at least about 1.2:1 and :not more than about 2.2:1, removing spent nitrating mixture from the resulting nitric acid esters of cellulose, and recovering directly, without subsequent stabilization treatment, stable nitric acid esters of cellulose.

10. A continuous process for preparing nitric acid esters of cellulose which comprises continuously reacting cellulose with a nitrating mixture containing essentially between about 45% and about 80% nitric acid, between about 10% and about 34% magnesium nitrate, and between about 7% and about 21% water by weight, the ratio of magnesium nitrate to water being at least about 1.2:1 and not more than about 2.2:1, the ratio of said nitrating mixture to cellulose being at least about 20:1

by weight.

11. A process for preparing nitric acid esters of cellulose which comprises immersing cellulose in a bath of nitrating mixture containing essentially between about 45 and about nitric acid, between about 10% and about 34% magnesium nitrate, and between about 7% and about 21% water by weight, the ratio of magnesium nitrate to water being at least about 1.2:1 and not more than about 2.2:1, the ratio of said nitrating mixture to cellulose being at least about 20:1 by weight, and nitrating the cellulose at a temperature between about 15 C. and about 70 C.

12. A process for preparing nitric acid esters of cellulose which comprises immersing cellulose in a bath of nitrating mixture containing essentially between about 45 and about 80% nitric acid, between about 10% and about 34% magnesium nitrate, and between about 7% and about 21% water by weight, the ratio of magnesium nitrate to water being at least about 1.2:1 and not more than about 2.2:1, the ratio of said nitrating mixture to cellulose being at least about 20:1 by weight, nitrating the cellulose at a temperature between about 15 C. and about 70 C., removing spent nitrating mixture from the resulting nitric acid esters of cellulose, and recovering directly, without subsequent stabilization treatment, stable nitric acid esters of cellulose.

13. A process for preparing nitric acid esters of cellulose which comprises immersing cellulose in a bath of nitrating mixture containing essentially between about 45% and about 80% nitric acid, between about 10% and about 34% magnesium nitrate, and between about 7% and about 21% water by weight, the ratio of magnesium nitrate to water being at least about 13:1 and not more than about 2.0:1, said nitrating mixture being substantially free of undissolved magnesium nitrate, the ratio of said nitrating mixture to cellulose being at least about 20:1 by weight, nitrating the cellulose at a temperature between about 15 C. and about 70 C. for at least about 8 minutes and until nitration is substantially complete, and recovering uniformly substituted nitric acid esters of cellulose substantially completely soluble in acetone to form smooth, clear solutions.

14. A process for preparing nitric acid esters of cellulose which comprises immersing cellulose in a bath or: nitrating. mixture containing, essentially between about 45 and abut'f80%..nitric,acid, between about 1 "and 34% magnesium-nitrate, and between about 7% and about 21% Water by weight, "the ratio of magnesium nitrate to watenbeingatjleast about 13:1, and not more than about'2'.0:1, said nitra'ting mixture being substan- 'ti'al ly"free of undissolved magnesium nitrate, the ratioof said nitrating mixture tocellulose being atleast about 20:1 by weight, nitrating the cellulose at a temperature between about 1'5" "1C.:,and-abo.ut"70 C. ffor at least about '8 minutes .and until nitration is substantially corn .plete, remoVingspeut nitrating mixture from the resulting nitric acid .esters .of cellulose,.and recovering directly, without subsequent stabilization treatment, uniformly substituted stable nitric acidv esters of cellulose substantially completely .solublein acetone to form smooth, clear solutions.

15. A continuous process. for preparing nitric acid esters of cellulose which comprisescontinuously immersing a sheet of cellulose ina bath of nitrating mixture containing essentially betweenabout 45% .andvabout 80% nitric acid, between about.10% andabout 34% magnesium nitrate, and. between about 7% .and about 21% water bywe ight, the ratio of magnesium nitrate to water being at least about 1.3:1 audnot moretthan about 2.021, saidfnitrating mixture being substantially free of undissolvecl magnesium nitrate,.-the ratio-of said nitrating mixture to cellulose being at ,leastabout 20:1 by weight, continuously nitrating the cellulose at a. temperature between about 15 C. and about 70 C. for at least-about 8 minutes and until nitration is substantially complete,

continuously removing spentnitrating mixture from the resulting nitrated sheet, and continuously recovering directly, without subsequent-stabilization treatment, uni .forr'nly. substituted, stable ;nitric ,acid. esters ,of ,cellulose substantially completely soluble .in acetone to form smooth, clear solutions.

vastness v conti uo proc s fo prepar n nit i Qi t rs .o cel u o which, comp se .euntinuous m er ing a 'sheet .of cellulosein a bath of nitrating v rrnxti r e containing .es u ia ylb w en abou 4 an abou 80% nitric acid,.between about andHabQUt 34% magnesium -nitrate, and betweenabout 7%, and about 21% water by .weifght, the ratio of magnesi11 m uitrate to water being at least about 1.3.:1 and not more than about2.0:1, said nitrating mixture being substantially free of undissolved magnesium nitrate, the ratio of said nitrating mixtureto cellulose being at least about 2() :1 y e h ous y nitra ing h ll b eatatmperature between about C. and about 70 C., continuously removing spent nitrating mixture from the resulting nitrated sheet, ,and, continuously recoyering directly, uniformly substituted, stable nitric acid esters of cellulose substantially,completely soluble ,in.,;ltone to formsmooth, clear solutions.

17. A new nitric acid ester of cellulose having. a nitrogen oontent of from about 11% to about 13.5% by weight and being free ,of unstable sulfur compounds, said ester being the reaction product of cellulosewithan excess oyer theoretical stoichiometric requirements of 'nitrating mixture containing essentially between about and about 94%,nit1 ic acid between about 3.3% and 'about ,34 ma-gnesium nitrate,., and betweenabout 2.7% and about 21% water by weight, the ratio of magnesium nitrate, to water being at least about 12:1 and not more than,aboutf 2.2: 1.

:.-References 'Cited'in the filerof this patent -UNITEDSTATESPATENTS Caeser May -14, 1946 OTHER REFERENCES Heuser: Cellulose Chemistry, 1944, pages 183, 200, 

1. A PROCES FOR PREPARING NITRIC ACID ESTERS OF CELLULOSE WHICH COMPRISES REACTING CELULOSE WITH AN EXCESS OVER THEORETICAL STIOCHIOMETRIC REQUIREMENTS OF NITRATING MIXTURE CONTAINING ESSENTIALLY NITRIC ACID, MAGNESIUM NITRATE AND WATER, THE RATIO OF MAGNESIUM NITRATE TO WATER BEING AT LEAST ABOUT 1.2:1 AND NOT MORE THAN ABOUT 2.2:1 BY WEIGHT, AND SAID NITRIC ACID BEING PRESENT IN AN AMOUNT SUFFICIENT TO YIELD A NITRIC ACID ESTER OF CELLULOSE HAVING A NITROGEN CONTENT OF FROM ABOUT 11% TO ABOUT 13.5% BY WEIGHT. 